Improvements to the material selector

[D. G. Jagielski]

The new material selector is both better and worse than the old one. Better, because you don't have to add materials manually and have decent material previews. Worse, because it scans the ENTIRE PROJECT for material, some of which you may not want to ever use, because they're meant for other stuff, such as e.g. player or other models.

It would be nice if the directories the material selector scans for materials could be set on a whitelist basis (i.e. the dev must consciously put material directory on the scan list for the material view to pick up materials from that directory).